This invention pertains generally to a method for providing void-free, protective coatings for surfaces and in particular for optical surfaces used for lithographic applications and subject to high energy radiation fluxes. In addition to being substantially void-free on a molecular level, the coatings produced by the method described herein are self-terminating so that the coatings are typically less than about 20 .ANG. thick.
There is a need in current technology to protect critical surfaces from degradation including corrosion and contamination. As technology progresses, the amount of surface degradation that can be tolerated usually becomes smaller, and more difficult to achieve. This is particularly true for advanced or next generation lithography where the goal is to produce circuits whose critical dimensions are below 0.1 .mu.m. The capabilities of conventional photolithographic techniques have been severely challenged by the need for circuitry of increasing density and higher resolution features. The demand for smaller feature sizes has inexorably driven the wavelength of radiation needed to produce the desired pattern to ever-shorter wavelengths. As the wavelength of the applied radiation is made shorter the energy of the radiation becomes greater, to the point where the radiation can cause the decomposition of molecules adsorbed on or proximate to a surface to produce reactive species that can attack, degrade, or otherwise contaminate the surface.
While short wavelength (high energy) radiation can directly dissociate molecules, secondary electrons, created by the interaction of this radiation with surfaces, are the primary agents for molecular dissociation. Low energy (5-10 eV) secondary electrons are known to be very active in breaking chemical bonds by direct ionization of adsorbed molecules or by electron attachment, wherein a secondary electron binds to a molecule producing a reactive negative ion that then de-excites to a dissociated product. Any type of radiation (photons, electrons, ions, and particles) that is energetic enough to liberate electrons can create secondary electrons; typically, energies of about 4-5 eV are required. Consequently, radiation-induced contamination, i.e., contamination of surfaces by reactive species produced by secondary electrons originating from radiative interactions, will most certainly occur in lithographic processes that use energetic radiation such as: extreme ultraviolet lithography (photon energy.apprxeq.100 eV), projection electron lithography (electron energy.apprxeq.50-100 keV), ion beam lithography (ion energy&gt;10 keV), 193 nm lithography (photon energy.apprxeq.6.4 eV) and 157 nm lithography (photon energy.apprxeq.7.9 eV). Thus, the potential for contamination of critical lithographic components, such as masks and optical surfaces, and degradation of their operational capability is present in all the advanced lithographic processes. Moreover, to make circuits with critical dimensions below 0.1 .mu.m, the figure and smoothness of the lithographic optical elements must be maintained at the nanometer level and below. This requires mitigation of degredative processes at nearly the atomic level. Future manufacturing technology (particularly for the semiconductor industry) therefore will require the application of protective coatings that have the following attributes:
1) Any coating that is applied must be resistant to contamination processes in general, and "high energy" degradative processes in particular. For example, future lithographic manufacturing processes can be expected to employ ionizing radiation, which will produce highly reactive species that can attack the coating.
2) The coating must be void-free on the molecular level in order to provide protection against processes that produce damage on the molecular size scale. If the shape and roughness of a lithographic optic is to be maintained below 1 nm (10 .ANG.), molecular-sized degradation must be prevented.
3) The coating process must have a wide process window allowing coating application with a variety of techniques, under a variety of circumstances, and in as flexible a manner as possible.
4) Protection must be achieved with as thin a coating as possible, preferably with thickness below 20 .ANG.. In optical applications, thin coatings avoid undesirable radiation absorption that would lower manufacturing throughput. Thin coatings also maintain optic figures and roughness specifications, with accuracy at the nanometer level.
Current methods of producing coatings on surfaces do not satisfy these requirements. For example, 20 .ANG. thick coatings created by methods known in the art such as ion sputtering, arc deposition, laser ablation, or electrochemical plating usually possess voids of dimension .about.5-10 .ANG.. These voids arise because these techniques generate .about.20 .ANG. diameter particles that cannot coalesce completely at the molecular level. As a result, coatings based on these techniques allow molecular-sized corrosive species to penetrate to the critical surface and damage it.